The present invention relates to a finger structured MOSFET, and more particularly to a MOSFET suitable for an application of reducing noises.
It is highly requested of an analog system circuit required to have a low noise characteristic that high-frequency noises of a transistor used for the analog circuit be decreased.
The noises emitted from the transistor are mainly caused by a resistance component. That is, the resistance component, when an electric current flows therethrough, emits heat noises, thus becoming a noise source.
Generally, the transistor used in a high-frequency analog amplifier circuit has a large size for reducing an input impedance and ensuring a gain. In this case, however, a gate resistance increases, and on the contrary the noise characteristic declines.
Such being the case, a comb-shaped transistor is proposed as a MOSFET exhibiting the low noise characteristic.
FIG. 13 is a plan view illustrating a construction of a finger structured MOSFET which has hitherto been proposed. FIG. 14 is a sectional view thereof taken along the line D-D. An active region 4 taking a laterally elongate rectangular shape serving as a source or drain region, which is surrounded with an element isolation layer 3, is provided within a well 2 formed in the surface of a semiconductor substrate 1. A polysilicon layer 6 serving as a gate electrode is provided on the element isolation layer 3 and the active region 4 as well, and a geometry thereof is, as shown in FIG. 13, that a plurality of gates each taking a rectangular shape with a predetermined width (a gate length) and connected to each other to form a gate electrode, reciprocates up and down with respect to the active region 4 on the element isolation layer 3 as viewed in FIG. 13 while traversing the active region 4, thus assuming rectangular undulations (a finger structure). Then, an impurity is diffused into the active regions 4 disposed on both sides of the gates defined as transverse areas, whereby sources 4a and drains 4b are alternately formed in self-alignment to the gates.
With such a geometry being adopted, an apparent gate width of the transistor is increased while restraining a gate resistance.
Further, a substrate contact region 5 for taking out an electric potential of the well, is provided outwardly at a spacing from the active region 4.
Moreover, a first contact hole 8 for supplying the gate electrode with an electric signal and a second contact hole 9 for taking out the well electric potential, are formed penetrating a first inter-layer insulating layer 7 deposited on the semiconductor substrate 1, and are filled with a metal such as aluminum etc, thus forming a first layer wire 10 through patterning. Herein, the first contact hole 8 is positioned between the active region 4 and the well electric potential take-out region 5.
A second inter-layer insulating layer 11 deposited on the first inter-layer insulating layer 7 is formed with a third contact hole 12 corresponding to the first contact hole 8, and a second layer wire (a gate wire) 13 is formed on the second inter-layer insulating layer 11, which involves filling the hole 12 with a metal and undergoing the patterning. Note that refractory metal silicide is provided on the surfaces of the gate electrode and of the gate wire, whereby a much lower resistance thereof can be attained.
Herein, as illustrated in FIG. 14, a variety of parasitic capacitances and parasitic resistances might occur in the construction of the finger structured MOSFET in FIG. 13. For instance, it might be considered that there exist a capacitance Cgb between the gate polysilicon layer and the well, a substrate resistance R between the well and the well electric potential take-out substrate contact, and a capacitance Cpad between the gate wire and the substrate.
On the other hand, the high frequency noises are influenced by a distance between the substrate contact and the element region and by a size of the well.
FIG. 11 is a graph showing a relationship between a drain current and a noise factor NF when an operation frequency is 2 GHz, wherein what is marked with a black circle shows a characteristic in a conventional MOSFET of which a gate width is 200 .mu.m and a gate length is 0.25 .mu.m.
In the conventional finger structured MOSFET, the substrate contact is formed outwardly of the contacts of the active region and of the gate electrode, and hence a distance between the substrate contact and an impurity diffused region could not be sufficiently decreased. There was therefore a limit in reducing the noises.